[ios][STG] Add cells to Recent Activity with placeholder data

This CL
- adds RecentActivityLogCell used for each cell in the recent
activity. It contains title, description, user's icon, favicon.
- adds -populateItemsFromService: to RecentActivityMediator to
initialize an array of RecentActivityLogItem. It's called when a
consumer is set. Now, it uses placeholder data instead of the actual
data.

See screenshot for the current status: https://drive.google.com/file/d/11RKcRLixuSufEs7KfgfLkZdatgKcWj0c/view?usp=sharing

not enough functions to test the behavior. Tests will be added in
follow-up CLs

Low-Coverage-Reason: The implementation is not done yet and there are
Bug: 370897655
Change-Id: I4d6cefa55a0927e9cf5a5d77b893b3f45756495d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5920922
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369347}